求教PHP正则匹配有关问题

$str = "akk c";

$pattern="/(.)\1/";

echo preg_match($pattern,$str,$matches)."
";

var_dump($matches);

?>

$str = "akk c";

$pattern="/k+/";

echo preg_match($pattern,$str,$matches)."
";

var_dump($matches);

$str = "akk c";

$pattern="/(.)\\1/";

echo preg_match($pattern,$str,$matches)."
";

var_dump($matches);